A large American flag flying above the Leonard P. Zakim Bunker Hill Memorial Bridge broke loose Friday and fell on top of a passing vehicle, causing a minor crash, State Police said.

The 20-by-40-foot flag fell as the 2010 Toyota Camry was traveling in the southbound lane on Interstate 93, causing the driver, a 55-year-old man, to lose control, spin out, and strike a median, State Police said.

The driver was unharmed, but the car sustained minor damage to its bumper, State Police said.

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Highway crews recovered the flag, and traffic was eventually allowed to resume on all lanes, according to State Police.

Michael Verseckes, a spokesman for the Massachusetts Department of Transportation, said the flag tumbled onto the car because the rope hoisting it was frayed. He believes exposure to harsh weather, as well as age, contributed to the fraying.

The flag was up for the Fourth of July and was supposed to be taken down on Tuesday, Verseckes said.

In the future, the condition of the rope and the flag will be monitored more carefully, Verseckes said.